1. Field of the Invention
This invention generally relates to a multi-way loudspeaker system and in particular to a multi-way loudspeaker system comprised of an array of multiple drivers capable of achieving high-quality sound.
2. Related Art
High-quality loudspeakers for the audio frequency ranges generally employ multiple specialized drivers for dedicated parts of the audio frequency band, such as tweeters (generally 2 kHz-20 kHz), midrange drivers (generally 200 Hz-5 kHz) and woofers (generally 20 Hz-1 kHz). Because of the necessary spacing due to the physical size of the specialized drivers, which is comparable with the wavelength of the radiated sound, the acoustic outputs of the drivers sum up to the intended flat, frequency-independent response only on a single line perpendicular to the loudspeaker, usually at the so-called acoustic center. Outside of that axis, frequency responses are more or less distorted due to interferences caused by different path lengths of sound waves traveling from the drivers to the considered points in space. There have been many attempts in history to build loudspeakers with a controlled sound field over a larger space with smooth out-of-axis responses.
For example, D'Appolito has presented a geometric approach to eliminate lobing errors in multi-way loudspeakers—a configuration using a center tweeter and two woofers arranged symmetrically along a vertical axis. Several loudspeaker manufacturers have adopted that approach and have even expanded upon it by using arrays of symmetrically arranged midrange drivers and woofers around one or two center tweeters. D'Appolito designs and those of the manufacturers that have adopted D'Appolito's approach utilize passive or analog crossover circuits or digital filters that emulate analog filters in a digital domain. Analog or passive crossover circuits inevitably introduce phase distortion. Further, with this design, spacing is not optimum and in general too large to completely avoid out-of-axis aberrations from an ideal smooth response.
In an alternative solution, the basic design concept is to apply very steep, “brick-wall” finite impulse response (FIR) filters to avoid large transition bands, so that the errors become inaudible. However, the individual polar responses of the involved drivers may still be different at the transition point, leaving audible discontinuities. Thus, with this design solution, it may be difficult to achieve a prescribed, smooth polar behavior throughout the whole audible range.
In yet another alternative, Van der Wal suggests that logarithmically spaced transducer arrays can achieve a very well controlled directivity, approximately constant over a wide frequency range, in one dimension. Some embodiments of this technique are described in U.S. Pat. No. 6,128,395. Like the previously described techniques, this design technique is limited because (i) the logarithmic spacing is prescribed only according to a given formula; (ii) the filter design is only valid for a particular case and (iii) severe errors may occur if the actual spacing deviates from logarithmic spacing, which may be unavoidable due to physical dimensions of the drivers or due to design constraints. Further, the design is restricted to one type of drivers, i.e., full-range drivers, limiting the application to public address systems. Thus, a need still exists for a loudspeaker configuration and filter design that overcomes the limitations of the prior art by providing a loudspeaker system that can contain drivers of various physical dimensions and can achieve prescribed, constant directivity over a large area in both the vertical and horizontal planes.